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Astronaut Scott Kelly said his perspective from space reveals a thin, fragile atmosphere

His year-long mission includes monitoring his health and preparing for potential trips to Mars

(CNN)In one of his final interviews from the International Space Station, astronaut Scott Kelly said that the Earth's atmosphere "looks very, very fragile" and "like something that we need to take care of."

From Scott Kelly's Twiiter account: This is a good example of the air pollution that exists across large parts of Asia. #YearInSpace

Kelly will return to Earth in March, but spoke with CNN's Chief Medical Correspondent Dr. Sanjay Gupta via satellite. During his time at the space station, Kelly has remained active on social media and often shares his unique perspective of the planet by posting photos. Because of his spectacularly encompassing vantage point, Gupta asked Kelly how he would define the Earth's condition if it were a human body.

"There are definitely parts of Asia, Central America that when you look at them from space, you're always looking through a haze of pollution," Kelly said. "As far as the atmosphere is concerned, and being able to see the surface, you know, I would say definitely those areas that I mentioned look kind of sick."

He said he notices weather systems, such as tropical cyclones, in unexpected locations.

"When you look at the ... atmosphere on the limb of the Earth, I wouldn't say it looks unhealthy, but it definitely looks very, very fragile and just kind of like this thin film, so it looks like something that we definitely need to take care of."

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Kelly is currently in his 501st day in space over six different missions and the 321st day of his one-year mission aboard the International Space Station. He has now been in space longer than any other U.S. astronaut.

Photos:Scott Kelly's top shots from space

Photos:Scott Kelly's top shots from space

NASA astronaut Scott Kelly posted some stunning photos during his yearlong mission aboard the International Space Station. He put the images on his Twitter and Instagram accounts.

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Photos:Scott Kelly's top shots from space

Kelly shared this photo as he started a 10-day countdown to his return. "I'm going to miss the colors of Africa!" he wrote on February 22.

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Photos:Scott Kelly's top shots from space

On February 7, Kelly snapped this photo of Levi's Stadium, where Super Bowl 50 was being played in Santa Clara, California. "Got to see the #SuperBowl in person after all!" he said online. "But at 17,500 MPH, it didn't last long."

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Photos:Scott Kelly's top shots from space

On January 23, Kelly shared this photo of a massive blizzard passing over Chicago. "#Blizzard2016 gave us an impressive view below," he said. "Stay warm!"

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Photos:Scott Kelly's top shots from space

August 27: "Day 154. #Aurora's purple glow adds mystery to the nightscape. Good night from @ISS!"

August 29: "Color palette of the #Spanish coast is an appealing morning view. #GoodMorning from @ISS!"

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Photos:Scott Kelly's top shots from space

July 25: "#Goodevening #Japan. Showing @Astro_Kimiya how to take pictures of #Earth at night."

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Photos:Scott Kelly's top shots from space

April 6: "Australia. You are very beautiful. Thank you for being there to brighten our day."

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Photos:Scott Kelly's top shots from space

August 31: "Day 157. At the end of the day, #sunrise will come again. Good night from @space_station!"

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Part of his current mission includes conducting a twin health study comparing the mental and physical toll on Kelly's body in space with his twin brother, retired astronaut Mark Kelly, who is on Earth. The medical tests are measuring the impact of zero gravity on bone density, vision, the microbiome, blood, heart and cells, as well as the psychological impact on mood, stress and cognitive functions.

Kelly said he is doing well, although he feels like he's been up in space for a really long time and looks forward to getting home soon.

"I don't really have any surprises," Kelly said. "A lot of the research we do is imagery based. Just this morning, I was doing some ultrasound exams on my eyes, on my heart and a lot of that data is stuff that will be analyzed by scientists and researchers and it will take time well after I'm back before we have results. But from a kind of a subjective perspective, I kind of knew what to expect going into this because I've flown a long-duration flight before, so you know there's been little effects on my vision which I had last time. But overall, nothing alarming."

Kelly will splash down March 1, along with Russian cosmonaut Mikhail Kornienko, who has spent the year with him on the ISS after launching on March 27, 2015.

Kelly also talked about the passage of time on the ISS, how feasible a trip to Mars could be, his views on private space travel and even what he would ask the candidates during a debate. The following interview has been edited for clarity.

Gupta: One of the objectives for this mission, besides the twin study, was to look at how feasible a mission to Mars might be. If this were a mission to Mars, which takes nine months or longer, you'd in fact, just be getting started with that mission as opposed to getting near to the end. I'm curious, with all that you learned, that you've seen, do you think Mars is feasible and also, when we eventually maybe get there, do you think there's extraterrestrial life there or other places?

Kelly: Yeah, I think it's definitely feasible. I think there are certain challenges -- you know, the radiation environment between the Earth and Mars is something that we're going to have to figure out, because we get protection here on the space station although we get a lot more radiation than you do on Earth. You get much much more on your way to Mars, so that's a challenge.

The systems, the life support systems, are on one hand a challenge, but we have been maintaining these systems here for the last 15 years now. We can get resupply quicker, but a lot we have learned here from operating a space station will help us go to Mars. I think the biggest challenge, though, is the public support and the financial support from our government or other governments to do this, because I think, technically, pretty much anything we have ever put our mind to we have been able to accomplish. So I think the challenge is mostly from public and government financial support. As far as whether there is life there on Mars or whether there was actually ever life there, I don't know. It would be great to find out, though.

Gupta: Gravitational waves have been found, proving yet another one of Einstein's theories correct. Another one of his theories that has to do with the space-time continuum, which would dictate that you're aging more slowly than your twin brother, Mark, [who is] here is on Earth. Obviously, it's a miniscule difference, but I'm curious, does it feel different time-wise? Does time change for you in some way, being up there?

Kelly: We are in this environment that is like a laboratory in many ways -- in some ways, it is kind of like a submarine or a ship -- but one in which you cannot leave and one in which you know: I'm at work when I go to sleep, I'm at work when I wake up.

I've been here since March [2015]. It's not like the days seem to go by slower, but definitely the whole period of time seems like a long time. A year now seems longer than I thought it would be and so I definitely have an appreciation for certain things that freedom and being on Earth provide. It's different when you're in space, but I definitely think I would kind of relish my freedom more after this experience than maybe I did before.

Gupta: I'm curious about your views on private space travel. There's been a lot of energy and money, frankly, around that industry lately. As someone who's done it, been up there, what do you think? Do you support it? Do you encourage it?

Kelly: Absolutely. This is an amazing experience. This is an incredibly complex facility and by doing really hard things, we reap the benefits and commercial space [travel] is hard. So, by developing those rockets and systems to have more people visit and live in space for periods of time, there will be benefits to that. ... It is something that I definitely feel strongly about. It's great that we have these companies that are pursuing this, because then it lets NASA direct their energy and focus on the frontier, the places we haven't been to before. I think those are the kinds of things the government should be involved with and I think something NASA does very well.

Gupta: What would you ask our future president or presidential candidates? We have a series of debates coming up here on CNN and I want to give you the floor for a second here, commander, to ask a question of our next president.

Kelly: You know, I guess, as an astronaut, as a NASA astronaut, I would ... feel obligated to ask a space-related question and what their goals and their vision are for our space program. I think our history in space and the International Space Station and those other things we do have significant value to our economy, to who we are as a people, as a species, really. I would want to know [what] their vision is for NASA and how they would plan to execute that vision.